To transfer power between two DC power sources at different voltages, a two-way DC-DC converter having a buck and boost function between the two power sources is required. For example, a power source system of a hybrid car has batteries on the high voltage side and low voltage side, supplies power to a drive motor via an inverter from the high voltage side battery, and supplies power to a car control electronic device from the low voltage side battery. The two batteries are connected via the DC-DC converter to enable mutual transfer of power and the DC-DC converter performs a buck operation when supplying power from the high voltage side to the low voltage side, and in an inverse case, performs a voltage boost operation.
A DC-DC converter having a two-way power conversion function between high and low voltage direct currents like this, for example, is disclosed in Japanese Patent Laid-open No. 2002-165448. The converter connects the high voltage side and low voltage side via a transformer and has a switching circuit for converting power in the two directions of direct current and alternating current on both high and low voltage sides. A power conversion device for converting power in the two directions of direct current and alternating current is called a converter, an inverter, or other various names. Here, it is called a switching circuit. Meanwhile, in Patent Document 1, firstly, the high voltage side switching circuit has a single-phase power conversion circuit in which a switching device is full-bridge (H bridge) connected and a smoothing circuit. On the other hand, the low voltage side switching circuit has a switching rectifying unit and a current fed type single-phase power conversion circuit composed of a choke coil and a smoothing condenser. And, when buck mode, the high voltage side switching circuit is subject to phase shift control, thus a square wave AC voltage is applied to the primary side of the transformer, is dropped by the transformer, is full-wave rectified by the low voltage side switching circuit, and is smoothed by the choke coil and smoothing condenser. When the low voltage side switching circuit performs a rectifying operation in synchronization with the high voltage side switching operation, a highly efficient power conversion can be performed.
On the other hand, when boost mode, the two switches of the low voltage side switching circuit are alternately turned off, thus a current flows through the secondary side of the transformer using the energy storage function of the choke coil, and the core of the transformer is changed in the magnetic flux. An AC voltage component proportional to the changing of the magnetic flux is generated on the primary side, is full-wave rectified by a free wheel diode of the high voltage side switching circuit, and is smoothed by the smoothing condenser.